1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to dryer fabrics for the dryer section of a paper machine.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
The present invention relates specifically to the dryer fabrics used in the dryer section. The cylinders in a dryer section may be arranged in a top and a bottom row or tier. Those in the bottom tier may be staggered relative to those in the top tier, rather than being in a strict vertical relationship. As the sheet proceeds through the dryer section, it may pass alternately between the top and bottom tiers as it passes first around a dryer cylinder in one of the two tiers, then around a dryer cylinder in the other tier, and so on sequentially through the dryer section.
In order to increase production rates and to minimize disturbance to the sheet, single-run dryer sections may be used to transport the sheet being dried at high speeds. In a single-run dryer section, such as that shown in FIG. 5, a paper sheet 198 is transported by use of a single dryer fabric 199 which follows a serpentine path sequentially about dryer cylinders 200 in the top and bottom tiers. Additionally, a number of turning rolls may be used. These turning rolls may be solid or vented.
It will be appreciated that, in a single-run dryer section, the dryer fabric holds the paper sheet being dried directly against the dryer cylinders in one of the two tiers, typically the top tier, but carries it around the dryer cylinders in the bottom tier. The fabric return run is above the top dryer cylinders. On the other hand, some single-run dryer sections have the opposite configuration in which the dryer fabric holds the paper sheet directly against the dryer cylinders in the bottom tier, but carries it around the top cylinders. In this case, the fabric return run is below the bottom tier of cylinders. In either case, a compression wedge is formed by air carried along by the backside surface of the moving dryer fabric in the narrowing space where the moving dryer fabric approaches a dryer cylinder. The resulting increase in air pressure in the compression wedge causes air to flow outwardly through the dryer fabric. This air flow, in turn, forces the paper sheet away from the surface of the dryer fabric, a phenomenon known as “drop off”. Drop off can reduce the quality of the paper product being manufactured by causing edge cracks. Drop off can also reduce machine efficiency if it leads to sheet breaks.
Many paper mills have addressed this problem by machining grooves into the dryer rolls or by adding a vacuum source to the dryer rolls. Both of these expedients allow the air otherwise trapped in the compression wedge to be removed without passing through the dryer fabric, although both approaches are expensive.
Contemporary dryer fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a base fabric usually woven from monofilaments and may be single-layered or multi-layered. The yarns are typically extruded from any one of several synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts.
Fabrics in modern papermaking machines may have a width of from 5 to over 33 feet, a length of from 40 to over 400 feet and weigh from approximately 100 to over 3,000 pounds. These fabrics wear out and require replacement. Replacement of fabrics often involves taking the machine out of service, removing the worn fabric, setting up to install a fabric and installing the new fabric. While many fabrics are endless, many of those used today are on-machine-seamable. Installation of the fabric includes pulling the fabric body onto a machine and joining the fabric ends to form an endless belt.
In response to this need to produce fabrics in a variety of lengths and widths more quickly and efficiently, fabrics have been produced in recent years using a spiral winding technique disclosed in commonly assigned U.S. Pat. No. 5,360,656 to Rexfelt et al., the teachings of which are incorporated herein by reference.
U.S. Pat. No. 5,360,656 shows a fabric comprising a base fabric having one or more layers of staple fiber material needled thereinto. The base fabric comprises at least one layer composed of a spirally wound strip of woven fabric having a width which is smaller than the width of the base fabric. The base fabric is endless in the longitudinal, or machine, direction. Lengthwise threads of the spirally wound strip make an angle with the longitudinal direction of the fabric. The strip of woven fabric may be flat-woven on a loom which is narrower than those typically used in the production of paper machine clothing.
The base fabric comprises a plurality of spirally wound and joined turns of the relatively narrow woven fabric strip. The fabric strip is woven from lengthwise (warp) and crosswise (filling) yarns. Adjacent turns of the spirally wound fabric strip may be abutted against one another, and the spirally continuous seam so produced may be closed by sewing, stitching, melting, welding (e.g. ultrasonic) or gluing. Alternatively, adjacent longitudinal edge portions of adjoining spiral turns may be arranged overlappingly, so long as the edges have a reduced thickness, so as not to give rise to an increased thickness in the area of the overlap. Alternatively still, the spacing between lengthwise yarns may be increased at the edges of the strip, so that, when adjoining spiral turns are arranged overlappingly, there may be an unchanged spacing between lengthwise threads in the area of the overlap.
In any case, a woven base fabric, taking the form of an endless loop and having an inner surface, a longitudinal (machine) direction and a transverse (cross-machine) direction, is the result. The lateral edges of the woven base fabric are then trimmed to render them parallel to its longitudinal (machine) direction. The angle between the machine direction of the woven base fabric and the spirally continuous seam may be relatively small, that is, typically less than 10°. By the same token, the lengthwise (warp) yarns of the woven fabric strip make the same relatively small angle with the longitudinal (machine) direction of the woven base fabric. Similarly, the crosswise (filling) yarns of the woven fabric strip, being perpendicular to the lengthwise (warp) yarns, make the same relatively small angle with the transverse (cross-machine) direction of the woven base fabric. In short, neither the lengthwise (warp) nor the crosswise (filling) yarns of the woven fabric strip align with the longitudinal (machine) or transverse (cross-machine) directions of the woven base fabric.
A fabric having such a base fabric may be referred to as a multiaxial fabric. Whereas the standard fabrics of the prior art have three axes: one in the machine direction (MD), one in the cross-machine direction (CD), and one in the z-direction, which is through the thickness of the fabric, a multiaxial fabric has not only these three axes, but also has at least two more axes defined by the directions of the yarn systems in its spirally wound layer or layers. Moreover, there are multiple flow paths in the z-direction of a multiaxial fabric. As a consequence, a multiaxial fabric has at least five axes. Because of its multiaxial structure, a multiaxial fabric having more than one layer exhibits superior resistance to nesting and/or to collapse in response to compression during the papermaking process as compared to one having base fabric layers whose yarn systems are parallel to one another.
The present invention provides an alternative to typical woven dryer fabrics. The present invention is a non-woven dryer fabric produced directly from raw material stock. This approach allows for the incorporation of bulk material elements in the fabric and for greater design control of the operating characteristics of the fabric. Moreover, the present fabric may be produced using a spiral winding technique, similar to that discussed above, only replacing the strips of woven material with raw stock material elements.